The present invention is related to a boron-free glass composition for producing glass fibers. More particularly, a boron-free and fluorine-free glas composition for producing glass fiber is provided that can be substituted for "E" glass and "621" glass composition used for producing glass fibers.
The conventional glass composition used in producing glass fibers into continuous glass fiber strands are "E" glass and "621" glass. Both of these glasses are calcium-aluminum-borosilicate glasses characterized by a low alkali oxide content usually calculated as sodium oxide (Na.sub.2 O). "E" glass is generally described and claimed in U.S. Pat. No. 2,334,961. The glass composition of "E" glass is given below in Table 1 with the constituents being in weight percent:
TAble I ______________________________________ Ingredients Percent ______________________________________ SiO.sub.2 52-56 Al.sub.2 O.sub.3 12-16 CaO 16-19 MgO 3-6 B.sub.2 O.sub.3 9-11 ______________________________________
The "621" glass is a modification of a typical "E" glass formulation as is shown above in Table I and is typically devoid of magnesium oxide (MgO) and has a calcium oxide (CaO) content that is much higher than that usually found in an "E" glass. The "621" glasses are described in more detail in U.S. Pat. No. 2,571,074. The glass composition of the "621" glasses is presented in Table II where the percentages shown are by weight:
TABLE II ______________________________________ Ingredients Percent ______________________________________ SiO.sub.2 52-56 Al.sub.2 O.sub.3 12-16 CaO 19-25 B.sub.2 O.sub.3 8-13 ______________________________________
It is well known in the art that both "E" and "621" glasses contain minor constituents which are typically F.sub.2, Fe.sub.2 O.sub.3, K.sub.2 O, Na.sub.2 O, S.sub.r O, and MgO, and on occasion BaO. In general the minor constituents are present each in amounts of less than 1 percent by weight in the glass.
The "E" and "621" glass compositions have been so readily accepted in the industry for producing glass fibers, because these compositions can be melted and refined at high rates and relatively low temperatures, have a low alkali content to avoid any hygroscopic problems, and have suitable electrical insulation properties. These glass compositions also have a workable viscosity over a wide range of relatively low temperatures, a low liquidus temperature range, and a low devitrification rate. Generally these glass compositions allow operating temperatures for producing glass fibers around 2,250.degree. F. to 2,500.degree. F. (1232.degree. C. to 1372.degree. C.) where the liquidus temperature is approximately 2,200.degree. F. (1205.degree. C.) or lower. It is advantageous to maintain a fiber forming temperature around 100.degree. F. (55.5.degree. C.) greater than the liquidus temperature in order to avoid devitrification.
In these compositions the silica is the basic glass former and boron oxide, which is also a glass former, is used as a flux. Calcium oxide (CaO), and alumina (Al.sub.2 O.sub.3) are added to assist in reducing any hygroscopic problems. Generally the boron or any fluorine containing compounds are present in the composition to reduce the viscosity of melt. The B.sub.2 O.sub.3 and F.sub.2 are volatile constituents of both glass compositions. The actual amount of the volatile constituents lost during the melting depends on several factors such as melting temperature, the rate of melting and water content of the melt. The component fluorine is provided by compounds such as fluorspar. The losses of fluorine occuring during glass melting will be of the order of one-half the fluorine added. The fluorine is lost by evolution of fluorine-containing compounds like SiF.sub.4.
The amount of fluorine-containing compound that is added contributes significantly to the corrosion of furnace refractories. In addition, the evolved fluorine-containing compounds pose a pollution problem to the environment surrounding the source of the evolved fluorine. For these reasons fluorine-free "E" glass compositions have been developed. An example of such a composition is given by K. L. Lowenstein, in "The Manufacturing Technology of Continuous Glass Fibers," Elsevir Scientific Publishing Company, New York, 1973 at page 30. The composition is given in Table III below in weight percents.
TABLE III ______________________________________ Ingredient Percent ______________________________________ SiO.sub.2 52.0 Al.sub.2 O.sub.3 14.6 B.sub.2 O.sub.3 8.8 MgO 2.0 CaO 21.5 BaO 0.6 Na.sub.2 O 0.1 K.sub.2 O 0.2 Fe.sub.2 O.sub.3 0.3 ______________________________________
Mr. Lowenstein notes that the problems of not extending the melting and homogenization rates of glass and not raising the liquidus temperature above 1140.degree. C. must be resolved before a fluorine-free glass composition becomes standard. The component B.sub.2 O.sub.3 can be provided by Colemanite, boric acid or boric oxide. The losses of B.sub.2 O.sub.3 occurring during glass melting are likely to be considerable and can range from 15 to 25 percent of the B.sub.2 O.sub.3 in the melt. Also in recent years the cost of the sources of B.sub.2 O.sub.3 has risen steadily, causing overall costs in the manufacture of glass fibers to increase substantially. While all costs have in general increased, the boric acid constituent has had a particularly significant effect on the increased cost of glasses for the manufacture of continuous glass fibers. In addition, it has been recently appreciated that since large quantities of B.sub.2 O.sub.3 are volatilized in the melting process much of the B.sub.2 O.sub.3 finds its way to the off gas stack and escapes into the environment.
The amount of F.sub.2 and B.sub.2 O.sub.3 escaping into the environment can be reduced by the use of cold top electric melt furnaces rather than furnaces heated by natural gas or by the installation of off gas stack cleaning apparatus. Both of these alternative approaches for reducing the amount of F.sub.2 and B.sub.2 O.sub.3 escaping into the environment are costly. Therefore, in order to reduce the cost of producing glass fibers and to reduce environmental pollution during the production of glass fibers without increasing the cost of production, a glass composition is needed that does not contain F.sub.2 and B.sub.2 O.sub.3 but still retains the favorable properties of "E" glass such as softening point, liquidus temperature, and tensile strength.
One approach for removing boron and fluorine from glass compositions used in producing glass fibers but still retaining the favorable properties of "E" glass for the production of glass fibers has been to substitute titanium dioxide (TiO.sub.2) and lithium oxide (Li.sub.2 O) for boron and fluorine as the fluxing agents in fiberizable glass compositions. Such an approach is described in the following U.S. Pat. Nos. 3,847,627; 3,847,626; 3,876,481 and 4,026,715. In this approach the glass fiber produced from the composition containing titanium dioxide has an unacceptable color. To overcome this handicap additional additives have been included in the glass composition. These additives include zinc oxide (ZnO), strontium oxide (SrO), and barium oxide (BaO). Also in some cases the amount of alkali oxide, (sodium oxide, potassium oxide, and lithium oxide) has been increased in amounts up to 3 percent by weight sodium oxide and potassium oxide, calculated as sodium oxide in equivalent molecular weight percent and up to 2 percent by weight of lithium oxide.
It is an object of the present invention to provide a glass composition free of boron with the addition of few if any additional constitutents to the glass composition but still retaining the favorable fiberizing properties of "E" or "621" glass compositions.
It is a further object of the present invention to provide a glass composition free of boron and fluorine but with similar and acceptable "E" or "621" glass composition properties of softening point, liquidus temperature, and tensile strength.
It is an additional object of the present invention to provide a glass composition free of boron and fluorine similar to "E" and "621" glass compositions in properties and components without the addition of a number of additional components to the composition and while retaining a low alkali content.